At present, with the issuing of increasingly strict environmental regulations by governments of due to increasingly serious environmental-protection situations, as well as obstinately high international oil prices and booming internet and electronic products, the battery market, such as hybrid electric vehicles, electric vehicles, rechargeable energy storing systems and uninterruptible power supplies for data centers, is brought to the new and rapid growth. The continuous booming of hybrid electric vehicles (HEV), pluggable hybrid electric vehicle (PHEV), and electric vehicles (EV) presses for a battery product capable of meeting the demands for high power, high energy storage, high reliability and safety, and environmental friendliness. The prevailing batteries in the past, i.e., lead-acid batteries and nickel-cadmium batteries (NiCd), can neither satisfy the demands for the development of the market, nor meet the demands for environmental protection any longer. Lithium-ion batteries are successfully applied in portable electronic devices; however, they have the disadvantages of insufficient power, high cost, and safety risk, failing to be applied in large-size systems. The newly emerging nickel-zinc batteries can meet all of the afore-mentioned demands: high power, sufficient energy (up to 4 times of that of lead-acid batteries), free from environment pollution (containing no lead, cadmium, or mercury), high reliability and safety (non-flammability), low cost, long life, and rechargeability.
A nickel-zinc battery includes a battery case, an electrode assembly, and electrolyte. The electrode assembly and the electrode are received in the battery case, and the electrode assembly includes a nickel positive electrode, a zinc negative electrode, and a membrane separator disposed between the nickel positive electrode and the zinc negative electrode. The existing nickel-zinc secondary battery normally directly adopts the nickel electrode used in a nickel-hydrogen battery or a nickel-cadmium battery, which is a flexible foil electrode formed by generally using a substrate of a nickel foil or foamed nickel, and coating the surface or inside of the substrate with positive electrode active material, that is, Ni(OH)2.
During the preparing and charging/discharging process of Ni(OH)2, there are always some unreduced Ni (III) ions, which are referred to as electron defects in the field of semiconductor, and some stoichiometrically excessive O2− ions, which are referred to as proton defects. Therefore, some amount of OH− ions in the lattice of Ni(OH)2 are replaced by O2− ions. The conductivity of this semiconductor not only depends on the motility of the electron defects and the concentration of the electron defects in the lattice, but also on the above defects existed in the lattice. These reasons lead to poor conductivity of nickel hydroxide, as well as the phenomenon that oxygen is evolved soon after being charged, penetrated the membrane separator, and adsorbed on the negative electrode. Therefore, Co2+ is often added to current nickel electrode to improve the conductivity of the nickel electrode, so that the reaction product Co3+ can form an excellent conductive net between the particles of nickel hydroxide with the proceeding of charge, so as to improve the “overpotential for oxygen evolution” of the nickel electrode, reduce the resistance of the battery, delay the evolution of oxygen, and enhance charging efficiency. Similarly, it facilitates improving the discharging depth of the nickel electrode during discharging, thereby improving the discharging capacity of the battery. Additionally, it is often necessary to add Cd2+ to Ni(OH)2, so as to improve the electrolytically charging overpotential of the battery.
However, when the above nickel electrode components are used in a nickel-zinc electrode, Co2+, which is easily dissolved in the electrolyte KOH solution, will diffuse toward the negative electrode, and is rapidly reduced on the zinc electrode, and precipitating Co on the negative electrode due to low potential for hydrogen evolution. The precipitated Co will form a hydrogen-evolving corrosive primary battery together with the zinc on the negative electrode, causing the continuous evolution of hydrogen and the soaring of the pressure in the battery, and increasing the possibility of explosion and liquid leakage in the battery, leading to large safety risk. At the same time, with the continuous consumption of the negative electrode and the fading of the capacity, the charge retention property becomes poorer. Additionally, the Cd added will cause pollution to the surrounding environment and the human body, decreasing the environmental friendliness of the battery.